The present invention relates to filters for electrical signals, called transversal filters, which use the transfer of charges into a semiconductor. It more particularly relates to a filter which is programmable by digital data.
A charge transfer transversal filter usually comprises a semiconductor substrate covered by an insulant on which are placed electrodes. By the periodic application of given potentials the latter ensure the transfer of packets of electrical charges representing the signal to be processed. These electrodes are arranged parallel to one another and transversely with respect to the charge transfer direction. Certain of these electrodes are transversely split into two unequal parts and the quantities of charges arriving beneath said electrodes are read differentially, i.e., for example the upper part of said split electrodes is given the plus sign and the lower part the minus sign. This leads to a weighting of the signal by coefficients equal respectively to the ratio of the lengths of the two parts of the split electrodes, the differential signal constituting the output signal of the filter.
The advantages of this type of filter are mainly due to the characteristics of the charge transfer devices, i.e. the constructional simplicity and low consumption.
An important disadvantage is that the filtering coefficients are finally fixed during the manufacture of the filter.